PROJECT SUMMARY This proposal will investigate a novel form of potentiation discovered in opioid-sensitive inhibitory inputs to the VTA. Drugs of abuse cause cellular adaptations in regions of the reward pathway that may precede the transition to addiction. The ventral tegmental area (VTA), a dopamine neuron-rich midbrain region, is involved in mediating responses to opioids. Opioid exposure causes long-term potentiation (LTP) at excitatory synapses on dopamine neurons and also blocks LTP at inhibitory synapses (LTPGABA). The net result of these opioid- induced changes in synaptic strength is to increase dopamine neuron firing, which is thought to be rewarding. However, the VTA is a heterogeneous region that has several distinct excitatory and inhibitory inputs, as well as different projection targets and there are possibly other forms of plasticity that have yet to be uncovered. For instance, I recently found that low frequency stimulation (LFS) which usually weakens synaptic input, increases synaptic strength in a subset of inhibitory inputs to the VTA (LFS-LTPGABA) if the stimulating electrode is placed caudal to the VTA. The potentiating inputs activated caudally are opioid-sensitive and sometimes exceptionally large in amplitude. Most reports describing plasticity in the VTA use gross electrical stimulation of afferents without consideration of circuit specificity. However, different electrode placements can activate different subsets of afferents, meaning that electrical stimulation is too crude to describe plasticity of all VTA synapses. Given that separate, distinct VTA circuits mediate distinct behavioral responses, the consequences of LFS- LTPGABA depend on which circuits it happens in. Experiments in Aim 1 will identify the postsynaptic cell identity and circuit of afferent and efferent connections to the VTA involved in LFS-LTPGABA. The implications of strong inhibitory afferents to the VTA having opioid-sensitive LFS-LTPGABA is that opioids could dramatically affect dopamine neuron excitability by damping a powerful source of inhibition. Experiments in Aim 2 will characterize the ?large amplitude? inputs, specifically, if they originate from a single axon, have LFS-LTPGABA, and if their plasticity is blocked by opioids. Determining where synaptic strengthening happens is important for any future therapeutic targeting of this plasticity. Experiments in Aim 3 will determine the locus of plasticity of LFS-LTPGABA and its sensitivity to opioids. Results from this proposal will provide critical information about the presence of LFS-LTPGABA across different cell types and circuits, the existence of strong inhibitory synapses on VTA neurons, whether LFS-LTPGABA is a presynaptic or postsynaptic change, and if LFS-LTPGABA is blocked by opioids. These findings will lay the groundwork for future experiments looking at how opioids and inhibition interact in the VTA of whole animals.